在 Scala 中，是否存在隐式视图无法传播到其他隐式函数的情况？

Question

假设定义了一个名为 'summoner' 的 class，它能够从作用域中调用隐式视图：

  case class Summoner[R]() {

    def summon[T](v: T)(implicit ev: T => R): R = ev(v)
  }

我发现它大部分时间都有效，但也有一些情况不起作用，例如以下是使用 singleton-ops 库的（不太）简短案例：


import shapeless.Witness
import singleton.ops.+
import singleton.ops.impl.Op

  trait Operand {

    def +[
        X >: this.type <: Operand,
        Y <: Operand
    ](that: Y): Op2[X, Y] = {

      Op2[X, Y](this, that)
    }
  }

  object Operand {

    abstract class ProvenToBe[O <: Arity]()(implicit val out: O) extends Operand {}

    object ProvenToBe {

      implicit class Trivial[O <: Arity, T <: ProvenToBe[O]](
          val self: T
      ) extends Proof {

        override type Out = O

        override def out: Out = self.out
      }
    }
  }

  trait Proof extends Serializable {

    def self: Operand

    type Out <: Arity

    def out: Out
  }

  object Proof {

    trait Out_=[+O <: Arity] extends Proof {
      type Out <: O
    }

    trait Invar[S] extends Out_=[Arity.Const[S]] {

      type SS = S
    }
  }

  trait Arity extends Operand {}

  object Arity {

    trait Const[S] extends Arity {

      type SS = S
    }

    object Const {

      implicit class Same[S](val self: Const[S]) extends Proof.Invar[S] {
        override type Out = Const[S]

        override def out: Const[S] = self
      }
    }

    class FromOp[S <: Op]() extends Const[S]

    object FromOp {

      implicit def summon[S <: Op](implicit s: S): FromOp[S] = new FromOp[S]()
    }

    class FromLiteral[S <: Int](val w: Witness.Lt[Int]) extends Const[S] {}

    object FromLiteral {

      implicit def summon[S <: Int](implicit w: Witness.Aux[S]): FromLiteral[S] =
        new FromLiteral[S](w)
    }

    def apply(w: Witness.Lt[Int]): FromLiteral[w.T] = {

      FromLiteral.summon[w.T](w) //TODO: IDEA inspection error
    }

  }

  case class Op2[
      +A1 <: Operand,
      +A2 <: Operand
  ](
      a1: A1,
      a2: A2
  ) extends Operand {}

  object Op2 {

    implicit class ProveInvar[
        A1 <: Operand,
        A2 <: Operand,
        S1,
        S2
    ](
        val self: Op2[A1, A2]
    )(
        implicit
        bound1: A1 => Proof.Invar[S1],
        bound2: A2 => Proof.Invar[S2]
    ) extends Proof.Invar[S1 + S2] {

      override type Out = Arity.FromOp[S1 + S2]

      override def out: Out = new Arity.FromOp[S1 + S2]()
    }
  }

当尝试按原样使用隐式视图时：

  implicit val a = Arity(3)
  implicit val b = Arity(4)

  val op = a + b

  op: Proof // implicit view works

但是使用召唤师时：

  val summoner = Summoner[Proof]()

  summoner.summon(op) // oops

[Error] /home/peng/git/shapesafe/spike/src/main/scala/edu/umontreal/kotlingrad/spike/arity/package.scala:141: No implicit view available from edu.umontreal.kotlingrad.spike.arity.package.Op2[edu.umontreal.kotlingrad.spike.arity.package.Arity.FromLiteral[Int(3)],edu.umontreal.kotlingrad.spike.arity.package.Arity.FromLiteral[Int(4)]] => edu.umontreal.kotlingrad.spike.arity.package.Proof.
one error found

FAILURE: Build failed with an exception.

这个错误消息看起来很平淡，几乎类似于常见的隐式类型不匹配错误，但以前的用法已经排除了这种可能性。所以我的问题是：

这种行为的原因是什么？
你怎么知道的？

Answer 1

我告诉过你关于使用 reify、-Xlog-implicits 调试隐式以及手动解析 In scala 2 or 3, is it possible to debug implicit resolution process in runtime?

中的隐式

如果你打印树

import scala.reflect.runtime.universe._
println(reify{
  op: Proof
}.tree)

你会看到隐式转换是如何解决的

(App.this.Op2.ProveInvar(App.this.op)(((self) => Arity.this.Const.Same(self)), ((self) => Arity.this.Const.Same(self))): App.this.Proof)

确实，手动解决了

summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op)(t =>
  Op2.ProveInvar(t)(a1 => Arity.Const.Same(a1), a2 => Arity.Const.Same(a2))
)

编译但编译器本身找不到隐式转换

summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op) //doesn't compile

如果您打开 -Xlog-implicits，您将看到详细信息

Information: $conforms is not a valid implicit value for App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing] because:
hasMatchingSymbol reported error: type mismatch;
 found   : App.Arity.FromLiteral[3] => App.Arity.FromLiteral[3]
 required: App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing]
  summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op)

Information: Arity.this.Const.Same is not a valid implicit value for App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing] because:
hasMatchingSymbol reported error: type mismatch;
 found   : App.Arity.Const[Nothing] => App.Arity.Const.Same[Nothing]
 required: App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing]
  summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op)

Information: App.this.Op2.ProveInvar is not a valid implicit value for App.Op2[App.Arity.FromLiteral[3],App.Arity.FromLiteral[4]] => App.Proof because:
hasMatchingSymbol reported error: No implicit view available from App.Arity.FromLiteral[3] => App.Proof.Invar[Nothing].
  summoner.summon[Op2[Arity.FromLiteral[3], Arity.FromLiteral[4]]](op)

正如我在 When calling a scala function with compile-time macro, how to failover smoothly when it causes compilation errors? 中告诉您的那样，您不能总是使用隐式参数 (implicit ev: T => R) 检查是否存在隐式转换。有时隐式实例 T => R 的存在与隐式转换 T => R 的存在不同（并非所有隐式转换都是基于类型 class 的）。尝试替换

val summoner = Summoner[Proof]()
summoner.summon(op) //doesn't compile

与

summonImplicitView[Proof](op) //compiles

def summonImplicitView[B] = new PartiallyAppliedSummonImplicitView[B]

class PartiallyAppliedSummonImplicitView[B] {
  def apply[A](a: A): B = macro summonImplicitViewImpl[A, B]
}

def summonImplicitViewImpl[A: c.WeakTypeTag, B: c.WeakTypeTag](c: whitebox.Context)(a: c.Tree): c.Tree = {
  import c.universe._
  val tpA = weakTypeOf[A]
  val tpB = weakTypeOf[B]
  val view = c.inferImplicitView(tree = a, from = tpA, to = tpB, silent = false)
  q"$view($a)"
}

您也可以尝试从 question

输入 class ImplicitView

case class Summoner[R]() {
  def summon[T](v: T)(implicit ev: ImplicitView[T, R]): R = ev.instance(v)
}

val summoner = Summoner[Proof]()
summoner.summon(op) // compiles

但是这种类型 class 并不总是有效，因为它是基于类型的并且并非所有隐式转换都是基于类型的，它在隐式解析期间忽略 v 的值。

我想我终于找到了问题所在（如果我们修复它 Summoner 将在没有宏的情况下工作）。你又失去了类型优化。

case class Summoner[R]() {
  def summon[T](v: T)(implicit ev: T => R): R = ev(v)
}

val summoner = Summoner[Proof {type Out <: Arity.FromOp[3 + 4]}]() 

// or even
//val summoner = Summoner[Proof {type Out <: Arity.FromOp[3 + 4]; type SS = 3 + 4}]()

summoner.summon(op) //compiles

这就是 -Xlog-implicits 日志中有 Nothing 的原因。

我想我修复了你的代码。在编写逻辑时，您将隐式实例与隐式转换混合在一起。隐式转换很棘手。我建议只根据类型 classes (MyTransform) 编写逻辑，然后如果需要转换，则根据这些类型 class 定义它们 (myConversion) es.

// doesn't extend T => R intentionally
trait MyTransform[-T, +R] {
  def transform(v: T): R
}
implicit def myConversion[T, R](v: T)(implicit mt: MyTransform[T, R]): R = mt.transform(v)

case class Summoner[R]() {    
  def summon[T](v: T)(implicit ev: MyTransform[T, R]): R = ev.transform(v)
}

trait Operand {  
  def +[
    X >: this.type <: Operand,
    Y <: Operand
  ](that: Y): Op2[X, Y] = {    
    Op2[X, Y](this, that)
  }
}
object Operand {   
  abstract class ProvenToBe[O <: Arity]()(implicit val out: O) extends Operand {}    
  object ProvenToBe {   
    implicit def trivial[O <: Arity, T <: ProvenToBe[O]]: MyTransform[T, Trivial[O, T]] = self => new Trivial(self)

    /*implicit*/ class Trivial[O <: Arity, T <: ProvenToBe[O]](
      val self: T
    ) extends Proof {   
      override type Out = O
      override def out: Out = self.out
    }
  }
}

trait Proof extends Serializable {    
  def self: Operand
  type Out <: Arity
  def out: Out
}
object Proof {
  trait Out_=[+O <: Arity] extends Proof {
    type Out <: O
  }

  trait Invar[S] extends Out_=[Arity.Const[S]] {
    type SS = S
  }
}

trait Arity extends Operand {}
object Arity {
  trait Const[S] extends Arity {
    type SS = S
  }
  object Const {
    implicit def same[S]: MyTransform[Const[S], Same[S]] = self => new Same(self)

    /*implicit*/ class Same[S](val self: Const[S]) extends Proof.Invar[S] {
      override type Out = Const[S]
      override def out: Const[S] = self
    }
  }

  class FromOp[S <: Op]() extends Const[S]
  object FromOp {
    implicit def summon[S <: Op](implicit s: S): FromOp[S] = new FromOp[S]()
  }

  class FromLiteral[S <: Int](val w: Witness.Lt[Int]) extends Const[S] {}
  object FromLiteral {
    implicit def summon[S <: Int](implicit w: Witness.Aux[S]): FromLiteral[S] =
      new FromLiteral[S](w)
  }

  def apply(w: Witness.Lt[Int]): FromLiteral[w.T] = {
    FromLiteral.summon[w.T](w) //TODO: IDEA inspection error
  }
}

case class Op2[
  +A1 <: Operand,
  +A2 <: Operand
](
   a1: A1,
   a2: A2
 ) extends Operand {}
object Op2 {
  implicit def proveInvar[A1 <: Operand, A2 <: Operand, S1, S2](implicit
    bound1: MyTransform[A1, Proof.Invar[S1]],
    bound2: MyTransform[A2, Proof.Invar[S2]]
  ): MyTransform[Op2[A1, A2], ProveInvar[A1, A2, S1, S2]]
  = self => new ProveInvar(self)

  /*implicit*/ class ProveInvar[
    A1 <: Operand,
    A2 <: Operand,
    S1,
    S2
  ](
     val self: Op2[A1, A2]
   )/*(
     implicit
     bound1: A1 => Proof.Invar[S1],
     bound2: A2 => Proof.Invar[S2]
   )*/ extends Proof.Invar[S1 + S2] {
    override type Out = Arity.FromOp[S1 + S2]
    override def out: Out = new Arity.FromOp[S1 + S2]()
  }
}

implicit val a = Arity(3)
implicit val b = Arity(4)

val op = a + b

op: Proof // compiles

val summoner = Summoner[Proof]()
summoner.summon(op) // compiles

在 Scala 中，是否存在隐式视图无法传播到其他隐式函数的情况？

In scala, are there any condition where implicit view won't be able to propagate to other implicit function?

scala

implicit

scalac

shapeless